Because acoustic ink printers (AIP) avoid the clogging and manufacturing problems of conventional drop-on-demand, nozzle-based ink jet printers, they represent a promising direct marking technology. While significant effort has gone into developing acoustic ink printing, see, for example, U.S. Pat. Nos. 4,751,530; 4,751,534; 5,028,937; and 5,041,849, problems remain.
An acoustic ink printer utilizes acoustic energy to eject droplets from an unbounded surface of a marking fluid onto a recording surface. Typically, this involves focusing acoustic energy from an ultrasonic transducer using either spherical or fresnel (reference U.S. Pat. No. 5,041,849) acoustic lenses into a focal area near the unbounded surface. If the acoustic energy is sufficient, an ink droplet (having a diameter about the size of the wavelength) is ejected. For a more detailed description of the ejection process reference is made to U.S. Pat. Nos. 4,308,547 and 5,028,937, and the citations therein.
As can be appreciated, acoustic ink printers are sensitive to the spacing between the acoustic energy's focal plane and the unbounded surface of the liquid. Since the focal plane is generally fixed, it is important that the unbounded surface be positioned near the focal plane. Indeed, since current practice dictates that the focal plane be within about one wavelength of the unbounded surface, and since typical wavelengths are about 10 micrometers, the location of the unbounded surface must be very accurately controlled. U.S. Pat. No. 5,028,937 discussed controlling the location of the unbounded surface using a perforated membrane. However, that solution may not be optimum.
It would be beneficial to have a device that accurately controls the location of the unbounded surface of a liquid, that is producible at low cost, and that allows droplets to be ejected onto a recording medium.